The microstructure of reduced activation ferritic/martensitic (RAFM) steels exposed to D plasma with different seeding impurities

EUROFER, P92 steel and iron samples were exposed in the linear plasma generator PSI-2 at a sample temperature of about 470–500 K with an incident ion flux of about 3–5 × 1021 m−2 s−1, an incident ion fluence of 1 × 1026 m−2 and an incident ion energy of 60–70 eV. Samples were exposed to deuterium plasma and with additional seeding impurities of He, Ar, Ne, Kr or Kr + He. Laterally averaged surface W enrichment varied between 0.6 and 6 at.%, depending on the exposure conditions, measured by energy dispersive x-ray spectroscopy with low energy electron beam and Rutherford backscattered spectroscopy. Microstructure observation revealed a complex morphology depending on the plasma composition. W enrichment was mostly located in the spike nano structures. Addition of He to the plasma rounded and enlarged the spikes on the surface whereas addition of heavier species to the plasma resulted in smoothing the steels surface. In case of steel samples exposed to D + He plasma, fine nano-bubbles with sizes below 3 nm were found near the sample surface. Sputtering rate increases by one order of magnitude by Ar and Ne seeding and by two orders of magnitude by Kr seeding for both types of steels. Measured D retention increases with He addition and decreases with higher-Z species seeding